Conventionally, septic tanks have been used as a means for reducing the biological oxygen demand of waste water (sewage) from residences and commercial establishments when a conventional public sewer connection to a municipal treatment plant is not available. These septic tanks are conventionally buried underground and influent waste is reduced by an anaerobic digestion process. Typically, the tank is divided into two compartments so that primary anaerobic treatment takes place in the first compartment, and reduced waste flows into the second compartment. After further anaerobic reduction in the second compartment, the waste is discharged through an effluent pipe into a subsurface drainfield from which it percolates into the surrounding soil. Usually, state and local regulations govern the waste strength of the effluent at discharge to subsurface drainfields. Thus, tanks must be sized to meet these environmental regulations.
With increasing population density in regions that are not supplied with conventional sewer connections to a municipal treatment plant, increasing attention is being paid to the environmental and public health risks posed by septic tanks. It is expected that certain municipalities may in the future require more stringent standards for the release of effluent from septic tanks. These regulations are expected to control the concentration of waste strength released from the septic tank, and the concentration of waste strength at various locations within the plume of effluent percolating away from the tank into the subsoil. Advantageously, certain local governments allow a smaller drainage field if the septic tank effluent meets the standard of discharging a "sand filter quality effluent" to the drainage field. Thus, if the efficiency of septic tank waste reduction can be improved to meet this requirement,, home and business land owners could potentially save money by having smaller lots, where lot size is dictated by drainfield requirements. This also allows potentially higher density housing, a benefit to land developers.
Such regulations may set standards that are difficult or impossible to meet with already-installed septic tanks. As a result, the regulations may encourage the installation of conventional sewer systems and municipal waste treatment plants. This solution is expensive, requiting large capital outlays, and is not practical in those areas where housing density is insufficient to economically support a conventional system. Consequently, other means must be found for further reducing the BOD of effluents from residential and commercial septic tanks.